sharing my experience not with shaving but with a different part of jaw 'reduction' (front upper / lower jaws pushed in about 10mm) you will experience sagging if you remove bone. you better expect to need a lift if you want to reverse that. Think about your bedsheets; they cover your bed nice and tight if you wrap the matching bedsheets around the matching mattress size. But if you start shrinking that mattress size or the sheets get worn out and loose with age, it wont fit as tight over the mattress.
As we age not only do our skin lose laxity and sag due to gravity but also our bones resorb and we lose bone density that holds up some of our facial muscles and other structures. So if you get any accusulpt or shaving, you will experience sag! It's not just the skin that causes sag; Another good example is anyone that's lost weight and has excess skin.